1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus that images the inside of a subject by using a magnetic resonance phenomenon, and a control method of the magnetic resonance imaging apparatus. The present invention particularly relates to a technology for easily setting a table on which a subject is placed to a target position.
2. Description of the Related Art
A magnetic resonance imaging method is a method of acquiring chemical and physical microscopic information about a substance by using a magnetic resonance phenomenon. The magnetic resonance phenomenon is a phenomenon that when being placed in a magnetic field, an aggregation of subject nuclear spins resonates with a radio-frequency magnetic field in which each atomic nucleus spins at a particular frequency (resonance frequency) responding to its own unique magnetic moment and an existing magnetic field, and generates a signal (magnetic resonance signal) in a relaxation process.
A magnetic resonance imaging apparatus using such magnetic resonance imaging method needs to position a receiving coil for receiving a magnetic resonance signal and the center of a magnetic field that is to be a reference when applying a gradient magnetic field. In other words, matching the center of the receiving coil to the center of the magnetic field is a requirement to obtain a highly accurate Magnetic Resonance (MR) image.
The reason for this is because a static magnetic field generated by a magnet and a gradient magnetic field generated by a gradient magnetic-field coil are both very difficult to be made spatially wholly uniform; uniformity of the static magnetic field and linearity of the gradient magnetic field are made generally ideal at the center, and getting more degraded with the periphery approaching, so that an image taken in the vicinity of the periphery has unsatisfactory fat suppression due to ununiformity of the static magnetic field and image distortion caused by non-linearity of the gradient magnetic field. Such degradation in the image quality often influences a diagnosis if exceeding a certain level.
Conventional positioning of a receiving coil and a table on which a subject is placed, for example, in a case of a coil used as a receiving coil for an abdomen, is performed by a method that, to begin with, the center of the receiving coil is placed on a subject's portion to be scanned, and furthermore, the scan portion is positionally adjusted with a projection light emitted by a projector for positioning, and then the table is moved to the center of a magnetic field. The method needs doubled operations and confirmations, so that it requires time and efforts.
Particularly, a table moving operation of matching a light emitted by the projector with a target position is difficult to perform through only one time of execution of start and stop of a move. Therefore, usually, it is required complicated operations of performing rough setting first, and then switching the movement speed of the table to a low speed, and repeating a fine adjustment of the table forward and backward. If using a projector that uses a laser light, when eyeballs of a subject pass under the projector, an attention needs to be made, for example, blocking out the projector or instructing the subject to close his/her eyes, in order to protect eyeballs.
If performing a scan by using a coil including a plurality of elements, such as a phased array coil; in order to perform the scan at the most sensitive position for the receiving coil element, and to prevent interference of any signal from an unwanted site, the positions of the coil elements to be used need to be accurately matched with the center of the magnetic field.
To solve such problems, for example, JP-A H7-124135 (KOKAI) discloses a method according to which a marker that generates an MR signal is provided at the center of a receiving coil; the position of the marker is obtained by collecting an MR signal from the marker, and then positioning of the receiving coil is performed. FIG. 15 is a schematic diagram for explaining a conventional method of positioning a receiving coil by using a marker. As shown in the figure, according to the method, an MR signal is collected by applying a readout gradient magnetic field GR [Hz/cm] of which a magnetic field in the z-axis direction is zero at the center of the magnetic field. As a result, when the frequency of the MR signal collected from the marker is f0+Δf [Hz] (f0 is the center frequency), a distance d between the center of the magnetic field and the marker is obtained by d=(f0+Δf−f0) /GR [cm].
Moreover, JP-A 2005-124855 (KOKAI) discloses a method of inserting a table on which a subject is placed to a desired position by using a combination of a light emitting element mounted on the upper surface of a receiving coil and a light receiving element mounted at the opening of a gantry. Furthermore, JP H5-65179 (KOKAI) discloses a unit that sets a region of interest on an image displayed on a monitor, and selects one of Radio Frequency (RF) coils. Moreover, JP-A 2007-282735 (KOKAI) discloses a method according to which a light emitting unit for guiding a subject is provided on a receiving coil in use, so that a subject can appropriately follow an instruction from an operator.
However, JP-A H7-124135 (KOKAI) does not propose any unit for an operator to select easily an element to be set at the center of the magnetic field when the receiving coil includes a plurality of elements. Moreover, according to the method disclosed in JP-A 2005-124855 (KOKAI), when a receiving coil includes a plurality of elements, or a plurality of receiving coils are simultaneously installed, a slide amount of the table cannot be set in accordance with the position of an element to be used from among them.
Furthermore, according to the method disclosed in JP H5-65179 (KOKAI), a scan position cannot be specified before collecting any image, so that it is impossible to specify the position of the table prior to a scan, so that an object to eliminate time and effort required in a procedure using a projector when inserting the table cannot be achieved. Moreover, JP-A 2007-282735 (KOKAI) does not propose any unit for the operator to select a receiving element of the RF coil prior to the insertion of the table.